Select resistant soybean varieties to manage disease
This year we have seen several soybean stem and root rot diseases for which the main management action is the use of resistant or tolerant varieties. Everyone reading this article does not need to use a resistant soybean variety in all their fields, but many farmers would benefit from using them to manage specific disease problems.
If you have a history of any of the problems mentioned below and a major area in a field is affected, make this a criteria in variety selection this year. There are enough resistant varieties on the market that you should be able to find varieties with the total genetic package you need. Unlike other traits that you pay more to have in your variety, disease resistance generally does not cost any more than comparable susceptible varieties.
Soybean Cyst Nematode (SCN)
In recent years we have seen an increase in the number of fields with SCN. Producers are encouraged to use the free testing program administered by UNL and funded by the Nebraska Soybean Board to determine if a field has SCN. If you have a history of SCN and you have been using resistant varieties, make sure you are rotating the varieties in the field and if at all possible, rotate the source of resistance.
There are three main genetic sources for resistance to SCN: PI88788, Peking (PI54840), and Hartwig (PI437654), marketed as CystX. Of these, PI88788 is found in the vast majority of SCN-resistant soybean varieties. As most readers will be using a PI88788 source of resistance, it is important to at least rotate the variety being grown in the field, as all PI88788 varieties are not the same.
For more information see the UNL Extension NebGuide, Soybean Cyst Nematode: Identification and Management (G1383).
Phytophthora Root and Stem Rot
This disease has been very active the last two years. Early planting in wet conditions creates a situation very conducive for Phytophthora if it’s present. Dry conditions followed by heavy rains have also been observed to result in significant disease levels in fields with a history of Phytophthora. In most cases symptoms develop within a week of a heavy rain. In addition to using resistant varieties, make sure you use an increased rate of mefenoxam or metalaxyl seed treatment for these fields. Note that seed treatments will only be effective for about 30 days and this disease can infect and kill the soybean plant throughout the season – so resistance is key to management.
Resistant Varieties: Using resistant varieties is the most effective means of managing Phytophthora rot of soybean. Genetic resistance is expressed in the host in terms of Rps (Resistant to Phytophthora sojae) genes. The genes are denoted as Rps 1a, 1b, 1c, 1d, 1k, 3, 6, and 7. The pathogen exists in races that interact with these genes. A race is identified by its interaction with the eight known Rps genes. The predominant races in Nebraska are 3 and 33. The most widely available resistance genes in the Midwest are 1c and 1k, commonly referred to as c or k in seed company literature. Gene 1c protects against races 1, 3, 8, 9, 13, 23, 28, 41, and 44 where gene 1k protects against races 1, 3, 4, 5, 8, 9, 13, 14, 18, 23, 43, and 44. Gene 3 is the only gene that protects against 99% of the races that occur in Nebraska.
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